Introduction

Wastewater
effluents from textile industries are increasingly complex to treat because
they contain important varieties of synthetic organic dyes, especially azo
dyes, following the increasing demand of manufacturing processes. Azo dyes are
found in discharges since their fixation rates on textiles remains below 60%.
They are known to be toxic at low concentration and, therefore, can cause
significant harm to both flora and fauna if they are not removed by appropriate
wastewater treatment. Conventional treatment processes are
coagulation-flocculation, adsorption and biological treatments, but also, and
increasingly, electrochemical processes that are of great interest in the
industry because, in the present scenario, they are becoming an alternative for
wastewater treatment for replacing the traditional methods, especially using
renewable electricity generation.

Electrocoagulation/electroflotation
(EC/EF) is a non-specific electrochemical water treatment technology that can
be applied to both drinking water and industrial liquid effluents. It presents
the particularity to produce a three-phase gas-liquid-solid system in which both
the gas and the solid phases contribute to pollution removal:

·      The solid phase, formed by
aluminium hydroxide, acts as an adsorbent and then settles;

·      The gas phase promotes the
adsorption of pollutants and hydrophobic solid particles on bubbles, followed
by flotation.

This
work deals with the modeling and the simulation by CFD (Computational Fluid Dynamics)
of aluminium electrodissolution coupled to current distribution and to hydrodynamics
in order to define an EC/EF cell with an optimized geometry for minimizing the operating
costs of treatment. The simulations are used to calculate the local flow by solving
the Navier-Stokes turbulently with the k-epsilon model, but also the primary and secondary current distributions;
they are validated using experiments carried out on an EC/EF cell operated
continuously.

Theoretical
background

The role of the current distribution is
critical because it determines the electrodissolution of the anode. In the
absence of mass transfer limitation, the uniformity of the current distribution
is a function of the Wagner dimensionless number:

     

 is the slope
of anodic polarization (Ω.cm²), K the electrolyte conductivity (Ω^-1 cm^-l)
and L the characteristic length of the system (cm).  

Primary current distribution (Wa<<1)

It is observed when the overpotential on
the electrodes is negligible. This implies that there are no significant
concentration gradients in the solution and no influence of the kinetics of reactions
at the electrodes. The primary current distribution only depends on the
geometry of the system [1]. It is calculated from the Laplace equation of the
electric potential:

 Â Â Â Â Â Â Â Â 

The integration of this equation provides
the spatial distribution of potential when the boundary conditions are known. Generally,
an electrochemical cell is completely enclosed by at least an anode, a cathode
and insulated walls on which (a)  and (b)  potential or
current are constant on the surface of the electrodes. The first condition
expresses that no electrical current passes through an insulating wall, the
second that the electrode interface is operated either in the potentiostatic or
the galvanostatic mode, respectively. The actual distribution of current is deduced
from that of the potential, and is calculated by means of Ohm's law

     

So we have just to calculate the gradient of
potential that is perpendicular to the electrode to define the current density
[2].

Secondary current distribution (Wa>>1)

This takes into account of the activation voltage which is a
function of the kinetics of the electrode reactions. The overpotential of concentration
is neglected. This is justified when the electrolysis current is small in
comparison with the limiting current. The activation overpotential adds an
equalizing effect on the current distribution, since the charge transfer
resistance at the interface becomes large in comparison to the resistance of
the solution [3]. The calculation principle remains the same as the primary
distribution. However, the boundary condition of the equation is no longer
valid because of the potential jump at the interface that must be known.

Material and methods

The
electrocoagulation cell operating in the continuous mode is composed of a
reactor equipped with nine rectangular aluminum electrodes in parallel
monopolar connection. Four cathodes and five anodes are alternatively placed in
the cell and connected to a DC generator and the distance between two consecutive
electrodes is 1.5 cm. The cathodes are fixed on one side and the cathodes on
the other side of the cell. In this electrode configuration, electrodes act as
baffles in the electrochemical cell, which results in a zigzag flow in the
latter during electrocoagulation. In turn, this has a benefic effect on mass
transfer and hydrodynamic processes. All dimensions are the same for electrodes
in terms of thickness (1 mm), width (9 cm) and height (11 cm), but the active
surface of each electrode is 9×9 cm² since the outlet is placed at 9
cm reactor from the bottom (FIGURE 1).

FIGURE 1. EC cell

Numerical study

Modelling and simulations have been carried out using the Comsol
Multiphysics software package. Simulations are performed by inserting three
physical models, namely the current distribution (including the kinetics which
is described as a Tafel term), the Navier-Stokes equations coupled to the
physical k k-epsilon model of turbulence, and the transport of concentrated
species whose distribution model is Fick?s law, while including transport
mechanisms such as migration in the presence of an electric field and
convection. The mesh which has been selected is a fine unstructured mesh which
is refined close to the electrodes. The number of elements of the boundary
layer has a growth rate of about 1.2 and an adjustment in the order of factor
5. FIGURE 2 shows the mesh used in the simulations.

FIGURE 2. Meshed
EC cell

Results

The results show
that the calculations converge rapidly when the cell is simulated in the galvanostatic
mode. The comparison between the primary and secondary current distributions highlights
that the activation overvoltage at the cathode standardizes the potential
distribution, as expected. The secondary current distribution is more
realistic, especially since the Tafel term is known to play an important role
in cell voltage with aluminum electrodes. The secondary distribution of the potential
is shown in FIGURE 3. Equipotential lines are parallel to electrodes, with a
linear decrease between anode and cathode, i.e. in the regions where
cathode and anode are facing, as expected at high current when Ohm?s law
prevails. The shape of the equipotential curves also remains unchanged
regardless of the applied current.

FIGURE 3. Secondary potential distribution in the cell (V)

The spatial distribution of
current is nearly uniform between the electrodes, with a peak close to the edge
of the electrodes and a sharp decrease next to the wall. This means that
current and potential distributions behave nearly ideally in the EC cell,
except close to the edge, in the inlet region and in the outlet region in which
current falls rapidly and can be neglected as a rough approximation.

The velocity magnitude at mid-height in the
EC cell is illustrated by FIGURE 4. The results show that a plug flow approximation applies
between each pair of electrodes, but that short-circuiting is observed in the
outlet region, coupled with dead zones, which presents the advantage to favor
flotation in these regions.

FIGURE
4. Spatial evolution of velocity magnitude (m/s) at mid-height in the EC cell

Simulations including gas and solid
formation have been driven, although gas hold-up remains small, except in the
foam at the cell surface. To compare experiments and simulations, the power
input and the amount of aluminium released by the electrolysis have been
measured by determining electrode mass loss. The results from simulations in FIGURE
5 illustrate a run with current intensity equal to 5.4A with a wastewater flow
rate of 0.5 L/min. Measurements after 20 min showed a mass loss of 793.5 mg on
the electrodes, that corresponded to 2.93 mol/m³ total aluminium in
the 10 L treated. The simulations demonstrate that under steady state
conditions, the amount of aluminium reached 3.12 mol/m³ at the
outlet of the EC cell. This highlights a good agreement between experiments and
simulations that has been confirmed by the comparison of power input. The small
difference between both values probably derives from the period in which steady
state is not achieved in the experimental work, which is not accounted by the
simulations.

FIGURE 5. Concentration of total aluminium (mol/m³)

Conclusions

The
methodology developed in this work shows that EC process can be effectively
described by CFD for pollution removal in terms of hydrodynamics, power input
and aluminium released. This constitutes a key result, as EC is considered as
an operation difficult to scale up because it is highly sensitive to the
distance between the electrodes and does not scale as hydrodynamic parameters.
This opens the opportunity for a better design of EC/EF cells that will improve
not only the performance, but also reduce the operating cost of this
technology. 

References

[1] R. Sautebin (1980), Modélisation
expérimentale et théorique du planage Anodique sous conditions d?usinage
Électrochimique, EPFL.

[2]
J.S. Newman, (1973), Electrochemical Systems, Prentice Hall.

[3] J.S.
Newman et C.W Tobias, (1962), J. Electrochem. Soc. 109, 1183

[4] A.
Storck, F. Coeuret, Élément de Génie Électrochimique, Lavoisier.